Lidar apparatus with an optical amplifier in the return path

ABSTRACT

A light detection and ranging (LIDAR) apparatus includes an optical circuit including a laser source configured to emit a laser beam, a beam separator operatively coupled to the laser source, the beam separator configured to separate the laser beam propagated towards a target, a first optical amplifier coupled to the beam separator, the first optical amplifier configured to receive a return laser beam reflected from the target in a return path and amplify the return laser beam to output an amplified return laser beam, and an optical component operatively coupled to the first optical amplifier, the optical component configured to output a current based on the amplified return laser beam.

TECHNICAL FIELD

The present disclosure relates generally to a light detection andranging (LIDAR) apparatus that provides an improved signal to noiseratio.

BACKGROUND

In an ideal case, typically the number of received photons (light) froma target is increased by sending more optical power (i.e., more photons)out or by enlarging the aperture size in the collection path. However,considering the limitations due to eye-safety of humans and preferencesin the size and cost of a LIDAR apparatus, neither of these methods isscalable.

Another way of increasing the signal to noise ratio (SNR) is by usingphotodetectors with high gain and high responsivity. However, such adetector or sensor (e.g., avalanche photodiodes) may be characterized bylow saturation optical power that may result in blinding the LIDARapparatus when it is detecting reflective objects and thereforedecreases the dynamic range of the sensor.

SUMMARY

The present disclosure includes, without limitation, the followingexample implementations. Some example implementations provide a lightdetection and ranging (LIDAR) apparatus comprising an optical circuitcomprising a laser source configured to emit a laser beam and a beamseparator operatively coupled to the laser source in which the beamseparator is configured to separate the laser beam propagated towards atarget. The optical circuit further comprises a first optical amplifieroperatively coupled to the beam separator. The first optical amplifieris configured to receive a return laser beam reflected from the targetin a return path and amplify the return laser beam to output anamplified return laser beam. The optical circuit further comprises anoptical component operatively coupled to the first optical amplifier inwhich the optical component is configured to output a current based onthe amplified return laser beam.

In accordance with another aspect of the present disclosure, an opticalcircuit for a light detection and ranging (LIDAR) apparatus comprises alaser source configured to emit a laser beam propagated towards a targetand a first optical amplifier operatively coupled to the laser source.The first optical amplifier is configured to receive a return laser beamreflected from the target in a return path and amplify the return laserbeam to output an amplified return laser beam. The optical circuitfurther comprises an optical component operatively coupled to the firstoptical amplifier in which the optical component is configured to outputa current based on the amplified return laser beam and a localoscillator signal.

In accordance with yet another aspect of the present disclosure, amethod comprises emitting, by a laser source, a laser beam, receiving,by an optical amplifier, a return laser beam reflected from a target ina return path, amplifying, by the optical amplifier, the return laserbeam to output an amplified return laser beam, mixing the amplifiedreturn laser beam with a portion of a local oscillator signal, andoutputting, by an optical component, a current based on the amplifiedreturn laser beam.

These and other features, aspects, and advantages of the presentdisclosure will be apparent from a reading of the following detaileddescription together with the accompanying figures, which are brieflydescribed below. The present disclosure includes any combination of two,three, four or more features or elements set forth in this disclosure,regardless of whether such features or elements are expressly combinedor otherwise recited in a specific example implementation describedherein. This disclosure is intended to be read holistically such thatany separable features or elements of the disclosure, in any of itsaspects and example implementations, should be viewed as combinableunless the context of the disclosure clearly dictates otherwise.

It will therefore be appreciated that this Brief Summary is providedmerely for purposes of summarizing some example implementations so as toprovide a basic understanding of some aspects of the disclosure.Accordingly, it will be appreciated that the above described exampleimplementations are merely examples and should not be construed tonarrow the scope or spirit of the disclosure in any way. Other exampleimplementations, aspects, and advantages will become apparent from thefollowing detailed description taken in conjunction with theaccompanying figures which illustrate, by way of example, the principlesof some described example implementations.

BRIEF DESCRIPTION OF THE FIGURE(S)

FIG. 1 illustrates a LIDAR apparatus according to exampleimplementations of the present disclosure.

FIG. 2 illustrates aspects of the optical circuits of the LIDARapparatus of FIG. 1 in accordance with an embodiment of the presentdisclosure;

FIG. 3 illustrates aspects of the optical circuits of the LIDARapparatus of FIG. 1 in accordance with an another embodiment of thepresent disclosure; and

FIG. 4 is a flow diagram of a method of operating a LIDAR apparatus inaccordance with an aspect of the present disclosure.

DETAILED DESCRIPTION

Frequency modulated continuous wave (FMCW) light detection and ranging(LIDAR) systems or apparatuses (such as coherent LIDAR systems)coherently mix light with two different delays which results in a beatradio frequency (RF) signal. When a target or object with lowreflectivity is far away from a LIDAR system, the reflected targetsignal from the target received by the LIDAR system may not havesufficient power for the LIDAR system to adequately detect the target.The sensing performance of LIDAR systems is improved when thesignal-to-noise ratio (SNR) is increased. For example, if the number ofcaptured photons reflected from the target is increased, the sensedsignal increases. The present disclosure describes an active method thatcoherently amplifies the optical target signal while maintaining a lownoise level. This method is robust, integrated optics compatible, and issuitable for mass manufacturing. The present disclosure describes acoherent LIDAR system architecture that takes advantage of coherentamplification of received photons from the target. This methodsignificantly improves the SNR of the beat RF signals, allowing objectswith lower reflectivity to be detected at a longer range. Aspects of thepresent disclosure improve the SNR of a LIDAR system by increasing thenumber of captured photons without sending more optical power toward thetarget or enhancing collection aperture/efficiency. In addition, thismethod is compatible with optical integrated circuits and can beimplemented in a small form factor.

FIG. 1 illustrates a LIDAR apparatus 100 according to exampleimplementations of the present disclosure. The LIDAR apparatus 100includes one or more of each of a number of components, but may includefewer or additional components than shown in FIG. 1. The LIDAR apparatus100 may be implemented in any sensing market, such as, but not limitedto, transportation, manufacturing, metrology, medical, and securitysystems. For example, in the automotive industry, the described beamdelivery system becomes the front-end of frequency modulatedcontinuous-wave (FMCW) devices that can assist with spatial awarenessfor automated driver assist systems, or self-driving vehicles. As shown,the LIDAR apparatus 100 includes optical circuits 101. The opticalcircuits 101 may include a combination of active optical components andpassive optical components. Active optical components may generate,amplify, or detect optical signals and the like. In some examples, theactive optical circuit includes lasers at different wavelengths, one ormore optical amplifiers, one or more optical detectors, or the like.

Passive optical circuits may include one or more optical fibers to carryoptical signals, and route and manipulate optical signals to appropriateinput/output ports of the active optical circuit. The passive opticalcircuits may also include one or more fiber components such as taps,wavelength division multiplexers, splitters/combiners, polarization beamsplitters, collimators or the like. In some embodiments, as discussedfurther below, the passive optical circuits may include components totransform the polarization state and direct received polarized light tooptical detectors using a PBS.

An optical scanner 102 includes one or more scanning mirrors that arerotatable along respective orthogonal axes to steer optical signals toscan an environment according to a scanning pattern. For instance, thescanning mirrors may be rotatable by one or more galvanometers. Theoptical scanner 102 also collects light incident upon any objects in theenvironment into a return laser beam that is returned to the passiveoptical circuit component of the optical circuits 101. For example, thereturn laser beam may be directed to an optical detector by apolarization beam splitter. In addition to the mirrors andgalvanometers, the optical scanning system may include components suchas a quarter-wave plate, lens, anti-reflective coated window or thelike.

To control and support the optical circuits 101 and optical scanner 102,the LIDAR apparatus 100 includes a LIDAR control systems 110. The LIDARcontrol systems 110 may function as a processing device for the LIDARapparatus 100. In some embodiments, the LIDAR control system 110 mayinclude signal processing 112 such as a digital signal processor. TheLIDAR control systems 110 are configured to output digital controlsignals to control optical drivers 103. In some embodiments, the digitalcontrol signals may be converted to analog signals through signalconversion unit 106. For example, the signal conversion unit 106 mayinclude a digital-to-analog converter. The optical drivers 103 may thenprovide drive signals to active components of optical circuits 101 todrive optical sources such as lasers and amplifiers. In someembodiments, several optical drivers 103 and signal conversion units 106may be provided to drive multiple optical sources.

The LIDAR control systems 112 are also configured to output digitalcontrol signals for the optical scanner 102. A motion control system 105may control the galvanometers of the optical scanner 102 based oncontrol signals received from the LIDAR control systems 110. Forexample, a digital-to-analog converter may convert coordinate routinginformation from the LIDAR control systems 110 to signals interpretableby the galvanometers in the optical scanner 102. In some embodiments, amotion control system 105 may also return information to the LIDARcontrol systems 110 about the position or operation of components of theoptical scanner 102. For example, an analog-to-digital converter may inturn convert information about the galvanometers' position to a signalinterpretable by the LIDAR control systems 110.

The LIDAR control systems 110 are further configured to analyze incomingdigital signals. In this regard, the LIDAR apparatus 100 includesoptical receivers 104 to measure one or more beams received by opticalcircuits 101. For example, a reference beam receiver may measure theamplitude of a reference beam from the active optical circuit, and ananalog-to-digital converter converts signals from the reference receiverto signals interpretable by the LIDAR control systems 110. Targetreceivers measure the optical signal that carries information about therange and velocity of a target in the form of a beat frequency,modulated optical signal. The reflected beam may be mixed with a secondsignal from a local oscillator. The optical receivers 104 may include ahigh-speed analog-to-digital converter to convert signals from thetarget receiver to signals interpretable by the LIDAR control systems110.

In some applications, the LIDAR apparatus 100 may additionally includeone or more imaging devices 108 configured to capture images of theenvironment, a global positioning system 109 configured to provide ageographic location of the system, or other sensor inputs. The LIDARapparatus 100 may also include an image processing system 114. The imageprocessing system 114 can be configured to receive the images andgeographic location, and send the images and location or informationrelated thereto to the LIDAR control system 110 or other systemsconnected to the LIDAR apparatus 100.

In operation according to some examples, the LIDAR apparatus 100 isconfigured to use nondegenerate laser sources to simultaneously measurerange and velocity across two dimensions. This capability allows forreal-time, long range measurements of range, velocity, azimuth, andelevation of the surrounding environment. In some exampleimplementations, the system points multiple modulated laser beams to thesame target.

In some examples, the scanning process begins with the optical drivers103 and LIDAR control system 110. The LIDAR control system 110 instructsthe optical drivers 103 to independently modulate one or more lasers,and these modulated signals propagate through the passive opticalcircuit to the collimator. The collimator directs the light at theoptical scanning system that scans the environment over a preprogrammedpattern defined by the motion control subsystem. The optical circuitsalso include a quarter-wave plate to transform the polarization of thelight as it leaves the optical circuits 101. A portion of the polarizedlight may also be reflected back to the optical circuits 101. Forexample lensing or collimating systems may have natural reflectiveproperties or a reflective coating to reflect a portion of the lightback to the optical circuits 101.

Optical signals reflected back from the environment pass through theoptical circuits 101 to the receivers. Because the light is polarized,it may be reflected by a polarization beam splitter along with theportion of polarized light that was reflected back to the opticalcircuits 101. Accordingly, rather than returning to the same fiber orwaveguide as an optical source, the reflected light is reflected toseparate optical receivers. These signals interfere with one another andgenerate a combined signal. Each beam signal that returns from thetarget produces a time-shifted waveform. The temporal phase differencebetween the two waveforms generates a beat frequency measured on theoptical receivers (photodetectors). The combined signal can then bereflected to the optical receivers 104. Configuration of opticalcircuits 101 for polarizing and directing beams to the optical receivers104 are described further below.

The analog signals from the optical receivers 104 are converted todigital signals using ADCs. The digital signals are then sent to theLIDAR control systems 110. A signal processing unit 112 may then receivethe digital signals and interpret them. In some embodiments, the signalprocessing unit 112 also receives position data from the motion controlsystem 105 and galvanometer as well as image data from the imageprocessing system 114. The signal processing unit 112 can then generatea 3D point cloud with information about range and velocity of points inthe environment as the optical scanner 102 scans additional points. Thesignal processing unit 112 can also overlay a 3D point cloud data withthe image data to determine velocity and distance of objects in thesurrounding area. The system also processes the satellite-basednavigation location data to provide a precise global location.

FIG. 2 illustrates aspects of the optical circuits of the LIDARapparatus of FIG. 1 in accordance with an embodiment of the presentdisclosure. An optical circuit, such as an optical circuit 200 shown inFIG. 2, can be part of optical circuits 101 shown in FIG. 1. The opticalcircuit 200 in FIG. 2 is shown as a side-view of a beam delivery system.Optical circuit 200 includes a laser source 202 configured to emit alaser beam, a beam separator (e.g., a beam splitter as shown) 204operatively coupled (for example, optically coupled) to laser source202, and a beam separator 208 configured to separate the laser beampropagated towards a target 214. Optical circuit 200 further includes afirst optical amplifier 216 operatively coupled to beam separator (e.g.,a polarization beam splitter as shown) 208, first optical amplifier 216configured to receive a return laser beam reflected from target 214 in areturn path and amplify the return laser beam to output an amplifiedreturn laser beam, and an optical component 224 operatively coupled tofirst optical amplifier 216. Optical component 224 is configured tooutput a current i_(PD) based on the amplified return laser beam. Lasersource 202 may suitably include a frequency modulated (FM) laser. Lasersource 202 may emit one or more lasers with different wavelengths andmay suitably include a single-mode or a multimode optical fiber.

Continuing with FIG. 2, optical circuit 200 further includes a beamsplitter 204, a second optical amplifier 206, a lens 210 to collimatethe light, and a polarization wave plate (PWP) 212 all in thetransmission or sending path of the laser beam. In one embodiment,polarization wave plate 212 may be a quarter-wave plate. A quarter-waveplate may transform the polarization to a circular polarization state.In another embodiment, polarization wave plate 212 may be a half-waveplate. A half-wave plate may shift the polarization direction oflinearly polarized light. Beam splitter 204 may suitably include a tapfiber optic beam splitter in one embodiment. Beam separator 208, lens210, and PWP 212 may suitably form a collimation optics. Second opticalamplifier 206 is in a transmission path of the laser beam propagatedtowards target 214 and second optical amplifier 206 is operativelycoupled between laser source 202 and beam separator 208. Beam splitter204 is operatively coupled between laser source 202 and first opticalamplifier 216, and beam splitter 204 is configured to output a localoscillation signal (LO) received as an input to optical component 224.First optical amplifier 216 may be a semiconductor optical amplifier(SOA) in accordance with an aspect of the present disclosure.Alternatively, first optical amplifier 216 may be a fiber opticalamplifier in accordance with another aspect of the present disclosure.Beam separator 208 may be a polarizing beam splitter (PBS) as shown inFIG. 2 in accordance with an aspect of the present disclosure.Alternatively, beam splitter 208 may be a circulator as shown in FIG. 3in accordance with another aspect of the present disclosure.

Continuing with FIG. 2, optical component 224 includes a beam combiner218 operatively coupled to an output of first optical amplifier 216,beam combiner 218 configured to receive a local oscillator signal (LO),and a photodetector 220 operatively coupled to beam combiner 218.Optical component 224 further includes an amplifier 222 operativelycoupled to photodetector 220 in which amplifier 222 is configured tooutput the current i_(PD). Beam combiner 218 may suitably include a50/50 fiber optic beam combiner, and amplifier 222 may suitably includea transimpedance amplifier (TIA) in one embodiment. In one embodiment,beam combiner 218 can be omitted from the return path so that the localoscillator path can be extracted from the reflection at the lens system210.

FIG. 3 illustrates aspects of the optical circuits of the LIDARapparatus of FIG. 1 in accordance with another embodiment of the presentdisclosure. An optical circuit, such as an optical circuit 300 shown inFIG. 3, can be part of optical circuits 101 shown in FIG. 1. Opticalcircuit 300 includes a laser source 302, a beam splitter 304, an opticalamplifier 306, a beam separator 308 which in the embodiment shown is acirculator, a lens 310, a target 314, an optical amplifier 316, and anoptical component 324. The optical component 324 includes a beamsplitter 318, a photodetector 320, and an amplifier 322 for outputtingcurrent i_(PD). The embodiment shown in FIG. 3 differs from theembodiment shown in FIG. 2 in that in FIG. 3, circulator 308 is usedinstead of a polarizing beam splitter and a polarization wave plate isnot used in conjunction with lens 310 to send laser beam P_(send) totarget 314. Otherwise, the embodiment shown in FIG. 3 is generally thesame as the embodiment shown in FIG. 2.

Continuing again with FIG. 2, laser source 202 emits a laser beam which,after undergoing processing via beam splitter 204, second opticalamplifier 206, beam separator 208, lens 210, and PWP 212, heads toward atarget such as target 214. The laser beam is reflected by the target andis received in the return path (“collection path”) as shown in FIG. 2.An optical amplifier (OA), such as first optical amplifier 216,amplifies the received photons (“reflected/returned laser beam”) from atarget, such as target 214, through stimulated emission. The duplicatephotons generated by the OA have the identical phase as the photonsreceived from the target. To quantitatively study the impact of gainmathematical equations that represent a linear frequency ramp in a FMCWLIDAR system may be utilized. The normalized electric field of opticalpower that is sent out by a FMCW LIDAR system is given by:

${e_{send}(t)} = {A_{send}{\cos\left( {{\omega_{0}t} + {\gamma\frac{t^{2}}{2}} + \phi_{0}} \right)}}$

Where A_(send) is the electric field amplitude of the power that leavesthe LIDAR, γ is the slope of frequency modulation, ω₀ is the carrierfrequency and ϕ₀ is the constant initial phase. The reflected power fromthe target that is collected by the lens system and is amplified by theOA is given by:e _(target)(t)=√{square root over (GRη)}×E _(Send)(t−τ)

where R is the reflectivity of the object (“target”), t is thecollection efficiency (determined by the lens system), and G is the OAgain.

A FMCW LIDAR system mixes the received power from the target with thelocal oscillator power (LO) that has an electric field of:

${e_{LO}(t)} = {A_{LO}{\cos\left( {{\omega_{0}t} + {\gamma\frac{t^{2}}{2}} + \phi_{1}} \right)}}$

in which A_(LO) is the amplitude of LO optical power. In this case thenormalized photocurrent due to mixing at photodetector (PD) is given by:

$\begin{matrix}{\mspace{79mu}{{{i_{PD}(t)} = {K \times \Re \times \left\langle {{{e_{LO}(t)} + {e_{Target}(t)}}}^{2} \right\rangle}}{{i_{PD}(t)} = {K \times \Re \times \left\lbrack {\frac{P_{LO} + {{GR}\;\eta\; P_{Send}} + P_{ASE}}{2} + {A_{LO} \times \sqrt{{GR}\;\eta}A_{send}{\cos\left( {{\omega_{0}\tau} + {{\gamma\tau}\; t} - {\gamma\frac{\tau^{2}}{2}}} \right)}}} \right\rbrack}}}} & ({e1})\end{matrix}$where K and

are the gain and responsivity of the PD respectively. Here P_(ASE) isthe amplified spontaneous emission power of the OA. In Equation 1 (e1),the oscillating term

$\left\lbrack {\sqrt{{GR}\;\eta}A_{send}{\cos\left( {{\omega_{0}\tau} + {{\gamma\tau}\; t} - {\gamma\frac{\tau^{2}}{2}}} \right)}} \right\rbrack$is the RF signal and the DC term

$\left\lbrack \frac{P_{LO} + {{GR}\;\eta\; P_{Send}} + P_{ASE}}{2} \right\rbrack$determines the shot noise. In one aspect, the LIDAR system is operatedat the shot noise limited region in order to achieve the maximum SNR. Inthis case the SNR can be calculated using time averaged signal:

$\begin{matrix}{{{SNR} = {\frac{\left\langle {i_{s}^{2}(t)} \right\rangle}{\left\langle {i_{shot}^{2}(t)} \right\rangle} = \frac{K^{2} + {\Re^{2} \times \left\langle {{A_{LO} \times \sqrt{{GR}\;\eta}A_{send}{\cos\left( {{\omega_{0}\tau} + {{\gamma\tau}\; t} - {\gamma\;{\tau^{2}/2}}} \right)}}}^{2} \right\rangle}}{2{qi}_{PD}{BW}}}}\mspace{20mu}{{SNR} = \frac{K \times \Re \times \left( {P_{LO} \times {GR}\;\eta\; P_{Send}} \right)}{q \times \left( {{P_{LO} \times {GR}\;\eta\; P_{Send}} + P_{ASE}} \right) \times {BW}}}} & {e2}\end{matrix}$where q is elementary charge and BW is the detection bandwidth. ForLIDAR systems, the received signal power is much smaller than the LO andASE powers (i.e. P_(LO) and P_(ASE)>P_(Send)). The SNR gain using the OAis approximately calculated by:

$\begin{matrix}{\frac{{SNR}_{OA}}{{SNR}_{{no}\mspace{11mu}{OA}}} \cong {\frac{P_{LO}}{P_{LO} + P_{ASE}} \times G}} & {e3}\end{matrix}$

Therefore, as shown in Equation 3 (e3), the SNR of a FMCW LIDAR systemusing the OA can be improved when P_(LO) is comparable to P_(ASE). Forinstance, for a commercial FMCW laser where P_(LO)˜P_(ASE) and eachpower level is approximately 1 mW and OA gain G can be up to 20 dB, aSNR gain of 17 dB may be achieved.

Note that Equation 2 (e2) indicates that the SNR can also be improved byincreasing the TIA gain (K), responsivity of PD (

), collection efficiency (η), or send optical power (P_(Send)), as wellas by decreasing the detection bandwidth (BW) (i.e increasing thedetection average time). Nevertheless, there is a limit for increasingthe TIA gain while achieving desired dynamic range and bandwidth.Responsivity of the detector can also be increased using avalanchephotodiodes (APDs) in accordance with an aspect of the presentdisclosure. Collection efficiency can be increased by enlargement of thecollection aperture; however, it is preferred to keep LIDAR systemscompact. Sent optical power should remain below a certain power in orderto guarantee requirements for eye safety. Bandwidth of detectiongenerally cannot be decreased, as it will increase the system latencyand slow down the sensor reaction. In one embodiment, optical circuits200 and 300 shown in FIGS. 2 and 3, respectively, may each be includedin a photonics chip or integrated circuit.

FIG. 4 illustrates an example flow diagram of a method 400 for operatinga LIDAR apparatus, according to some aspects of the disclosure. In someembodiments, the flow diagram 400 may be performed by one or morecomponents of the systems and apparatuses described with reference toFIGS. 1-3 above.

With reference to FIG. 4, method 400 illustrates example functions usedby various embodiments. Although specific function blocks (“blocks”) aredisclosed in method 400, such blocks are examples. That is, embodimentsare well suited to performing various other blocks or variations of theblocks recited in method 400. It is appreciated that the blocks inmethod 400 may be performed in an order different than presented, andthat not all of the blocks in method 400 may be performed. Flow diagram400 includes emitting, by a laser source, a laser beam at block 402. Theflow diagram 400 further includes at block 404, receiving, by an opticalamplifier, a return laser beam reflected from a target in a return path(reflected laser beam from the target) and at block 406, amplifying, bythe optical amplifier, the return laser beam to output an amplifiedreturn laser beam. The flow diagram 400 further includes at block 408,mixing the amplified return laser beam with a portion of a localoscillator signal and at block 410, an optical component outputs acurrent based on the amplified return laser beam. In embodiments, flowdiagram 400 may further include processing, by the optical component, alocal oscillator signal with the amplified return laser beam to outputthe current and amplifying, by another optical amplifier, the laser beambefore separating the laser beam propagated towards the target.

The preceding description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a good understanding of several embodiments of thepresent disclosure. It will be apparent to one skilled in the art,however, that at least some embodiments of the present disclosure may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented insimple block diagram format in order to avoid unnecessarily obscuringthe present disclosure. Thus, the specific details set forth are merelyexemplary. Particular embodiments may vary from these exemplary detailsand still be contemplated to be within the scope of the presentdisclosure.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiments included inat least one embodiment. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment” in various places throughout thisspecification are not necessarily all referring to the same embodiment.In addition, the term “or” is intended to mean an inclusive “or” ratherthan an exclusive “or.”

Although the operations of the methods herein are shown and described ina particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be in an intermittent oralternating manner.

The above description of illustrated implementations of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. The words “example” or“exemplary” are used herein to mean serving as an example, instance, orillustration. Any aspect or design described herein as “example” or“exemplary” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Rather, use of the words“example” or “exemplary” is intended to present concepts in a concretefashion. As used in this application, the term “or” is intended to meanan inclusive “or” rather than an exclusive “or”. That is, unlessspecified otherwise, or clear from context, “X includes A or B” isintended to mean any of the natural inclusive permutations. That is, ifX includes A; X includes B; or X includes both A and B, then “X includesA or B” is satisfied under any of the foregoing instances. In addition,the articles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Moreover, use of the term “an embodiment” or “one embodiment” or“an implementation” or “one implementation” throughout is not intendedto mean the same embodiment or implementation unless described as such.Furthermore, the terms “first,” “second,” “third,” “fourth,” etc. asused herein are meant as labels to distinguish among different elementsand may not necessarily have an ordinal meaning according to theirnumerical designation.

What is claimed is:
 1. A light detection and ranging (LIDAR) apparatus,comprising: an optical circuit comprising: a laser source configured toemit a laser beam; a beam separator operatively coupled to the lasersource, the beam separator configured to separate the laser beam into alocal oscillator and a transmission beam propagated towards a target; afirst optical amplifier operatively coupled to the beam separator, thefirst optical amplifier configured to: receive a return laser beamreflected from the target in a return path; and amplify the return laserbeam to output an amplified return laser beam, wherein a power level ofthe local oscillator is comparable to a power of amplified spontaneousemission of the first optical amplifier; and an optical detectoroperatively coupled to the first optical amplifier, the optical detectorconfigured to output a current based on the amplified return laser beamand the local oscillator.
 2. The LIDAR apparatus of claim 1, wherein thefirst optical amplifier comprises a semiconductor optical amplifier. 3.The LIDAR apparatus of claim 1, the optical detector comprising: a beamcombiner operatively coupled to an output of the first opticalamplifier, the beam combiner configured to receive a local oscillatorsignal; a photodetector operatively coupled to the beam combiner; and anamplifier operatively coupled to the photodetector, the amplifierconfigured to output the current.
 4. The LIDAR apparatus of claim 1,wherein the beam separator comprises a polarizing beam splitter, theLIDAR apparatus further comprising an optical amplifier positioned afterthe polarizing beam splitter.
 5. The LIDAR apparatus of claim 1, whereinthe beam separator comprises a circulator.
 6. The LIDAR apparatus ofclaim 1, the optical circuit comprising a second optical amplifier in atransmission path of the laser beam propagated towards the target, thesecond optical amplifier operatively coupled between the laser sourceand the beam separator.
 7. The LIDAR apparatus of claim 1, the opticalcircuit comprising a beam splitter operatively coupled between the lasersource and the first optical amplifier, the beam splitter configured tooutput a local oscillation signal received as an input to the opticaldetector.
 8. The LIDAR apparatus of claim 1, wherein the optical circuitis included in a photonics chip.
 9. An optical circuit for a lightdetection and ranging (LIDAR) apparatus, the optical circuit comprising:a laser source configured to emit a laser beam propagated towards atarget; a beam separator operatively coupled to the laser source, thebeam separator to separate the laser beam into a local oscillator signaland a transmission beam propagated towards a target a first opticalamplifier operatively coupled to the laser source, the first opticalamplifier configured to: receive a return laser beam reflected from thetarget in a return path; and amplify the return laser beam to output anamplified return laser beam, wherein a power level of the localoscillator signal is comparable to a power of amplified spontaneousemission of the first optical amplifier; and an optical detectoroperatively coupled to the first optical amplifier, the optical detectorconfigured to output a current based on the amplified return laser beamand the local oscillator signal.
 10. The optical circuit of claim 9,wherein the first optical amplifier comprises a semiconductor opticalamplifier.
 11. The optical circuit of claim 9, wherein the opticaldetector comprises: a beam combiner operatively coupled to an output ofthe first optical amplifier, the beam combiner configured to receive thelocal oscillator signal; a photodetector operatively coupled to the beamcombiner; and an amplifier operatively coupled to the photodetector, theamplifier configured to output the current.
 12. The optical circuit ofclaim 9, wherein the beam separator comprises a polarizing beamsplitter.
 13. The optical circuit of claim 9, wherein the beam separatorcomprises a circulator.
 14. The optical circuit of claim 9, furthercomprising: a second optical amplifier in a transmission path of thetransmission beam propagated towards the target, the second opticalamplifier operatively coupled between the laser source and the beamseparator.
 15. The optical circuit of claim 9, further comprising: abeam splitter operatively coupled between the laser source and a secondoptical amplifier, the beam splitter configured to separate a firstportion of the laser beam propagated towards the target and a secondportion of the laser beam as the local oscillator signal received at theoptical detector.
 16. A method comprising: emitting, by a laser source,a laser beam; separating, by a beam separator, the laser beam into alocal oscillator signal and a transmission beam propagated towards atarget; receiving, by an optical amplifier, a return laser beamreflected from a target in a return path; amplifying, by the opticalamplifier, the return laser beam to output an amplified return laserbeam, wherein a power level of the local oscillator signal is comparableto a power of amplified spontaneous emission of the optical amplifier;mixing the amplified return laser beam with a portion of a localoscillator signal; and outputting, by an optical detector, a currentbased on the amplified return laser beam and the local oscillatorsignal.
 17. The method of claim 16, further comprising: processing, bythe optical detector, a local oscillator signal with the amplifiedreturn laser beam to output the current.
 18. The method of claim 16,further comprising: amplifying, by another optical amplifier, the laserbeam, wherein separating the laser beam propagated towards the target isin response to amplifying the laser beam by the another opticalamplifier.
 19. The method of claim 16, wherein the optical amplifiercomprises a semiconductor optical amplifier.
 20. The method of claim 16,wherein the optical amplifier comprises a fiber optical amplifier. 21.The method of claim 16, wherein the beam separator comprises apolarizing beam splitter.